RFID systems typically include RFID tags and RFID readers (the latter are also known as RFID reader/writers or RFID interrogators). RFID systems can be used in many ways for locating and identifying objects to which the tags are attached. RFID systems are particularly useful in product-related and service-related industries for tracking large numbers of objects being processed, inventoried, or handled. In such cases, an RFID tag is usually attached to an individual item, or to its package.
In principle, RFID techniques entail using an RFID reader to interrogate one or more RFID tags. The reader transmitting a Radio Frequency (RF) wave performs the interrogation. A tag that senses the interrogating RF wave responds by transmitting back another RF wave. The tag generates the transmitted back RF wave either originally, or by reflecting back a portion of the interrogating RF wave in a process known as backscatter. Backscatter may take place in a number of ways.
The reflected-back RF wave may further encode data stored internally in the tag, such as a number. The response is demodulated and decoded by the reader, which thereby identifies, counts, or otherwise interacts with the associated item. The decoded data can denote a serial number, a price, a date, a destination, other attribute(s), any combination of attributes, and so on.
An RFID tag typically includes an antenna system, a power management section, a radio section, and frequently a logical section, a memory, or both. Circuitry for the RFID tag is conventionally implemented as an integrated circuit (IC) that is formed on a semiconductor die. Due to various reasons, such as manufacturing cost, power efficiency, and consumer demands, it is desirable to continually miniaturize the circuitry for the RFID tag. As a result, circuit designers are constantly seeking improved designs that allow for more efficient utilization of space on the semiconductor die on which the IC for the RFID tag is formed.
For example, in earlier RFID tags, the power management section included an energy storage device, such as a battery. RFID tags with an energy storage device are known as active tags. However, advances in circuit design and semiconductor technology have miniaturized the electronics so much that an RFID tag can be powered solely by the RF signal it receives. Such RFID tags do not include an energy storage device, and are called passive tags.
Passive tags typically include at least one of rectifier circuits and charge pump circuits for harvesting usable power from the RF signal received via the antenna system. In operation, an alternating RF wave received by the antenna system is converted by the rectifier circuits and charge pump circuits into usable direct current (DC) voltage that can be used to power the operation of the passive RFID tag.
Harvesting sufficient power from the RF wave can be difficult since the voltage of the RF signal is in the range of approximately 200 millivolts, and a typical supply voltage for circuits of the RFID tag is one volt. Additionally, for relatively high-voltage operations in the RFID tag, such as for programming and erasing non-volatile memory in the RFID tag, a boosted voltage as high as 12 volts may be needed. Complicating matters is that the RF wave received by the RFID tag is not constantly provided, and can cease to be transmitted by the RFID reader without any notice. Thus, operation of passive RFID tags converting the low-level RF waveform to a usable voltage requires rectifiers and charge pump circuits that can generate usable voltage quickly and efficiently.